REST: Resolve issues with filters

REST: Resolve issues with filters

Turns out filtering patches using a series string wasn't as easy as we
thought. We need to slugify State.name, but unfortunately that isn't
stored in the database. The tests were hiding this fact as State objects
created by 'tests.utils.create_state' don't have spaces in them.

Override custom versions of both django-filter's 'Filter' class and
the Django 'Form' required by this, and update the tests to prevent a
regression.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 6222574 ("REST: filter patches by state name")
Tested-by: Philippe Pepiot <philippe.pepiot@logilab.fr>